Converter ignores fields from child class when unstructuring (inheritance)

• cattrs version: current master (a10ad1c638dd16308c9aa73c649fa92282503c00)
• Python version: 3.10
• Operating System: Linux

Description

I'm serializing a dataclass with a field. If the field has a more generic type than the actual instance, all the information from the child class is lost when unstructuring.

What I Did

from dataclasses import dataclass

import cattr


@dataclass(kw_only=True, frozen=True)
class Bar:
    x: int


@dataclass(kw_only=True, frozen=True)
class Foo(Bar):
    y: int


@dataclass(kw_only=True, frozen=True)
class Baz:
    foo: Bar

converter = cattr.Converter()
foo = Baz(foo=Foo(x=1, y=2))
print("unstructured", converter.unstructure(foo))

Expectation vs Reality

I'd expect it to output unstructured {'foo': {'x': 1, 'y': 2}} but it outputs unstructured {'foo': {'x': 1}}

Is this expected behaviour due to the way Converter generates unstructuring functions? If so, how can you change that behaviour?

Start with the cattr.Converter().unstructure call and the dataclass inheritance example in the issue. Reproduce the output difference between Bar and Foo, then trace how unstructuring handles the declared Bar field versus the runtime Foo instance. Done means the example includes both x and y without breaking existing unstructuring behavior.